Tube lamps have been preferred over incandescent lamps on account of their efficiency. A tube lamp such as a fluorescent tube lamp operates from a ballast, which regulates the current through the lamp. Tube lamps have been developed for running on electromagnetic (EM) ballasts, and also on high-frequency (HF) ballasts. An electronic ballast for a fluorescent lamp basically fulfills three functions: it provides filament heating, it ignites the lamp, and it stabilizes the lamp current in steady state. The performance of an electronic ballast is generally superior to that of a magnetic ballast, since it can essentially eliminate flicker and can start the lamp faster.
LED lamps are even more efficient than fluorescent lamps, and are attractive on account of their long life and low energy consumption. Retrofit LED tube lamps have been developed for insertion into the housings of fluorescent tube lamp fixtures. In order to use a retrofit LED tube lamp (commonly referred to as an “LED tube”) with a legacy ballast, one option has been for a technician to rewire the fixture in case an HF ballast is present in the fixture. In recent years standardization bodies have commenced developing safety standards for “direct replacement” LED tubes, i.e. non-integrated or semi-integrated LED tube lamps for which the ballast is part of the lighting fixture, and for which the lighting fixture does not require modification or rewiring. Since the ballast is incorporated in the fluorescent lighting fixture, it is preferable to design the LED tube lamp to operate from that ballast. For this reason, the ballast of a fluorescent tube lamp fixture is referred to as a “legacy ballast” in the following. One widely used HF ballast design is the half-bridge LC resonant circuit, of which countless variations are possible. One of the challenges in the development of HF-compatible LED tubes is to ensure compatibility with many different kinds of ballast topologies. The variety of concepts based on the different topologies effectively makes it impossible to design a passive matching circuit for a LED tube that matches all HF-ballasts.
Therefore, it is an object of the invention to provide an improved LED tube lamp that is economical, efficient, and compatible with various different kinds of HF ballast.